Traditionally, steam generators have been designed with independent pressure parts that are bundled and enclosed in elaborate, engineered, shipping frames for shipping and handling. At the construction site, the pressure parts are set on a field-erected operating frame and the shipping frames are removed and discarded. The pressure parts are then enclosed by a field fabricated casing enclosure. Consequently, nearly all assembly as well as erection was accomplished in the field and this required the transportation and handling of a multitude of individual parts.
Additionally, such designs are typically bottom-supported designs which means that pressure parts are supported from lower headers that, in turn, rest upon upright columns positioned beneath the headers. Bottom-supported units require elaborate expansion hardware at the top of the unit to compensate for the large expansion differential that occurs between the pressure and non-pressure parts.
With a modular design of steam generator components, it is possible to maximize shop assembly and minimize field erection assembly and its associated higher costs. Current modular designs feature independent parallel flow casings having inner steam/water cooled heating surfaces and these independent casings result in separate parallel gas flow streams through the various modules. These separate flow streams prevent the gas from mixing which would otherwise result in desirable uniform gas temperatures. Consequently, considerable gas temperature variations may exist between these separate flow paths which adversely affect the heat transfer and temperature balance of the steam generator.